Natural gas, which consists primarly of methane (CH4), is an attractive energy source as it produces less nitrogen oxides (NOx) and carbon dioxide when burned than other energy sources such as oil or coal. Many natural gas-powered engines, as well as other types of engines, may operate under a “lean-burn” condition in which a lean fuel/air mixture is combusted to generate a mechanical output. A lean fuel/air mixture has an excess of air and a fuel/air ratio of less than one, whereas a rich fuel/air mixture has an excess of fuel and a fuel/air ratio of greater than one. The excess air in a lean fuel/air mixture may lead to reduced emissions of nitrogen oxides (NOx), as well as improvements in fuel efficiency as more oxygen is available to combust with the fuel.
Although effective, engines using lean fuel/air mixtures may be prone to unstable combustion due to pockets of air that are void of sufficient fuel molecules to provide continuous combustion reactions. As a result, lean-burn engines may suffer from unstable combustion which may involve drawbacks such as slow and/or incomplete combustion, or unreliable spark ignition of the lean fuel/air mixtures which could cause misfiring.
As a way to speed up combustion and improve the reliability of ignition, some lean-burn engines may include a pre-chamber to initiate combustion of lean fuel/air mixtures in the main combustion chamber (see U.S. Pat. No. 7,261,097). In particular, the pre-chamber may be a small chamber in fluid communication with the main combustion chamber in which a fuel-rich mixture is ignited by a spark plug. The ignited fuel mixture in the pre-chamber may produce a flame which may jet out into the main combustion chamber to ignite and initiate combustion of the lean fuel/air mixture in the main combustion chamber. Other technologies use free radicals to trigger ignition of lean fuel/air mixtures in combustion chambers, without a spark or flame as the ignition source.
However, there is a need for additional strategies for enhancing combustion stability in lean-burn engines, particularly to support efforts to develop engines designed to operate under leaner and leaner conditions. Among other things, the present disclosure addresses this problem.